Semiconductor devices are the key elements for the majority of electronic systems, including communications, consumer, data-processing, and industrial-control equipment. As discrete components, semiconductors have wide applications because they have a wide range of current- and voltage-handling capabilities and because they lend themselves to integration into complex but readily manufacturable microelectronic circuits.
Most semiconductor circuit chips are manufactured through a process called fabrication, a series of procedures through which semiconductor devices are formed in and on the surface of a polished wafer. Fabrication involves a sequence of procedures, including layering thin sheets of materials on the surface of the wafer, patterning to define the geometric features of the chip, doping, and annealing. The patterning process of fabrication entails creating a pattern on the surface of a wafer by utilizing photolithographic processes to transfer the desired pattern from a photomask to the surface of the wafer. An etching process may then be introduced to remove the external material of the wafer that is not covered by photoresist material.
In etching, material is selectively removed from the surface of the wafer in order to define the structure of a inwardly disposed layer. One method of etching, called plasma etching, uses gases in a plasma state to remove unwanted material from the surface of the wafer. The plasma etch process is accomplished by exposing the wafer to a gas plasma, which chemically reacts with the material to be removed and physically removes it.
Because it is impossible to etch materials located at different points on the surface of the wafer at exactly the same rate, it is extremely advantageous to utilize etching methods having high degrees of selectivity in the manufacturing process in that the processes are able to etch one material at a higher rate than another. Therefore, a need has arisen for a method that overcomes the disadvantages of existing processes and allows for a higher range of selectivity in the etching process. Although previous inventions have attempted to overcome this problem by utilizing variations of power modulation in plasma etching processes, such methods result in ranges of selectivity that continue to be somewhat limited.